The production of organic compounds using synthesis gas, which is a mixture of carbon monoxide and hydrogen, or from carbon monoxide as one of the reactants has been known for a significant period of time. It is well known that one can produce methanol directly from synthesis gas and that methanol can be further reacted by hydroformylation, homologation and carbonylation reactions to produce acetaldehyde, ethanol and acetic acid or its methyl ester, respectively. It is also known that alcohols, esters, ethers, and other organic compounds can be reacted with synthesis gas or carbon monoxide to produce oxygenated organic compounds. The difficulties, however, have resided in the ability to carry out any one of these chosen reactions to produce the desired compound at acceptable efficiency, conversion rate and selectivity.
In almost all instances the reaction is generally catalyzed using a Group VIII transition metal compound as the catalyst and a halogen as the promoter. It is known that many other metal compounds and promoters can be used. In addition, the prior art has disclosed the use of secondary activators or ligands in conjunction with the metal catalysts and promoters. These secondary activators can be other metallic salts or compounds, amines, phosphorus compounds, as well as a multitude of other compounds that have been disclosed in the published literature. Thus, a typical catalyst system contains the metal atom catalyst, promoter and, optionally, ligands, solvents and secondary activators. Though a significant amount of literature does exist describing the production of acetic anhydride, to our knowledge it does not disclose or suggest our invention. Several of the pertinent patents in this area are discussed below.
Typical prior art processes employing rhodium catalyst to carbonylate methyl acetate, require rather harsh reaction conditions of temperature and pressure to obtain satisfactory yields of products. Such reaction conditions require use of expensive reactors, engender excessive energy cost, often lead to undesired by-products and cause excessive corrosion problems.
Various patents have issued dealing with carbonylation of methyl acetate. In U.S. Pat. No. 4,251,458, issued Feb. 17, 1981, a carboxylic acid anhydride is prepared by carbonylating a carboxylate ester employing a catalyst consisting of Rh--Cr--CH.sub.3 I--ER.sub.3, wherein E is As and R is an organic moiety. Typical operating conditions for vapor-phase operation included employing reaction temperatures from 100.degree. to 350.degree. C., most preferably from 175.degree. C. to 225.degree. C., and reaction pressures from 1 to 5000 psia, most preferably from 150 to 500 psia. In the actual test runs reported in the examples the reaction temperature was 160.degree. C. and the reaction pressure was 750 psia. Accordingly, this process employs a different catalyst at different operating conditions then the present invention.
British Patent Specification No. 1,468,940 issued Mar. 30, 1977, to Halcon Research and Development Corporation discloses a catalyst for carbonylating methyl acetate to form acetic anhydride wherein the catalyst consists of Rh-Cr-CH.sub.3 I. The process is carried out at reaction temperatures of 175.degree. C. and reaction pressures on the order of 350 psia. The patent neither discloses nor suggests employing phosphine ligands as a key ingredient in the catalyst. In general, acetic anhydride was formed at the rate of less than about 1.25 moles per liter per hour. This process does not utilize the mild reaction conditions of the instant process.
British Patent Specification Nos. 2,067,556 ('556) and 2,067,557 ('557), both issued July 30, 1981 to Halcon Research and Development Corporation disclose preparation of acetic anhydride from methyl acetate in a carbonylation process employing a catalyst system different from that of the present invention. In the '556 Patent the catalyst was Rh--Hf--CH.sub.3 I--ER.sub.3, while in the '557 Patent the catalyst was Rh--Zr--CH.sub.3 I--ER.sub.3, wherein E is N, As, P and Sb. In the experiments reported in the Examples the reaction temperatures were on the order of 160.degree. C. and the reaction pressures were on the order of 700 psig. Although reaction temperatures as low as 100.degree. C. and reaction pressure as low as 15 psig were recited in the specification, no examples having such low temperatures or low pressures were provided.
In European Patent Application No. 055,622 published July 7, 1982, a catalyst system for the carbonylation of methyl acetate to acetic anhydride is disclosed. The catalyst includes nickel; a halide; the ligand ER.sub.3, wherein E is N, P, As, Sb and R is an organic moiety and a co-catalyst (N) which is a Group IA, IIA, IIIB or IVB metal. Operating temperatures can vary from 100.degree. to 250.degree. C., while operating pressures were from 40 to 2200 psig. The experiments reported in the examples were generally carried out at reaction temperatures from 180.degree. C. to 200.degree. C. and reaction pressures from 1000 to 1500 psig with acetic acid as a solvent. The ligand employed in most runs was 2, 4-lutidine. This process employs higher operating temperatures and pressures and a different catalyst than the present process.
European Patent Application No. 055,192, published June 30, 1982 teaches preparation of acetic anhydride by carbonylation of methyl acetate employing, as the catalyst system, nickel, methyl iodide; an ionic iodide; a carboxylate co-catalyst--LiOAc and a carboxylic acid solvent, such as acetic acid. The reactions were generally carried out at a reaction temperature of about 180.degree. C. and a reaction pressure on the order of 1000 psig. Accordingly, the catalyst and reaction conditions were different from those of the present invention.
In European Patent Application No. 055,970 published July 14, 1982 a process similar to that disclosed in European Patent Application No. 055,192 is disclosed, with the exception that the ionic halide component of the catalyst is R.sub.4 EI wherein E is N or P. The reaction is typically carried out at operating temperatures on the order of 180.degree. C. and operating pressures on the order of 1300 psig.
European Patent Application No. 067,777 published Dec. 22, 1982 is directed to the carbonylation of methyl acetate with a Co-Ru-MI-MOAc catalyst to obtain acetic anhydride. The halogen is specifically an ionic iodide. MI, as opposed to a covalent iodide, such as CH.sub.3 I. MI is typically a phosphonium salt, [R.sub.4 P]I, such as [CH.sub.3 (Ph).sub.3 P]I. A fourth component of the catalyst is a metal acetate, where the metal can be an alkali metal.
The reaction is typically carried out at reaction temperatures from 180.degree. to 210.degree. C. and at reaction pressures on the order of 3700 psig. There is no disclosure or suggestion of the instant novel rhodium complex catalyst. In addition, the actual operating parameters of temperature and pressure for that process are not the mild operating parameters of the present process.
European Patent Application No. 070,788 published Jan. 26, 1983 relates to the formation of acetic anhydride by the carbonylation of methyl acetate with a Co--Cr, Mo, or W--MI catalyst. MI represents an ionic iodide such as [CH.sub.3 (Ph).sub.3 P]I or an alkali, alkaline, lanthanide or actinide metal halide. The process described is similar to that illustrated in European Patent Application Nos. 067,777 and 070,787. The reaction is normally carried out at temperatures on the order of 210.degree. C. and at operating pressures on the order of 3700 psig. There is no disclosure or suggestion of the novel rhodium complex catalyst of the instant invention or of the ability to employ such catalyst at mild operating conditions.
In European Patent Application No. 070,180 published Jan. 19, 1983 there is disclosed a process for carbonylation of methyl acetate to acetic anhydride. The catalyst employed is Ni--I--Gp--IV/Gp I/Gp II catalyst, and a typical catalyst is Ni--CH.sub.3 I--Sn (OAc).sub.2 --LiOAc. While the application theoretically discloses that broad ranges of temperatures and pressures are possible, nonetheless all the actual runs reported are carried out at temperatures on the order from 175.degree. to 200.degree. C. and at operating pressures from 980 to 1400 psig. Accordingly, it is clear that both the catalyst and actual operating parameters are different from the far milder operating conditions and the unique rhodium complex catalyst of the present invention.
Japanese Patent Publication No. 57-176921, filed by Showa Denko is directed to a vapor phase process for the carbonylation of methyl acetate to acetic anhydride. The catalyst employed is rhodium supported on a porous carrier, such as activated carbon. The present catalyst is a different rhodium complex. Methyl iodide is co-fed with CO and methyl acetate. The range of theoretical operating temperature is generally said to be from about 200.degree. to 500.degree. C. and the preferred operating pressures are said to be from 14 to 450 psig. In the reported test runs, however, the operating temperatures of the relevant Examples ranged from 200.degree. to 230.degree. C. and the operating pressures were in the range from 150 to 220 psig. Such conditions are not the mild conditions of the instant process.
As shown above, numerous Group VIII metals and a halide promoter are known to catalyze the carbonylation of methyl acetate to acetic anhydride. For the most part such catalysts operate at relatively harsh reaction conditions of temperature and pressure. Rhodium is known to act at less harsh pressure conditions. To obtain reasonable reaction rates, however, higher operating temperatures of greater than 160.degree. C. and usually more than 180.degree. C., are required.